r root-hairs have crept, they have eaten into the
marble and left it corroded. The marks will become more distinct if the
marble is rubbed with a little vermilion.

In order that the processes of solution and absorption may take place, it
is necessary that free oxygen should be present. All living things must
have oxygen to breathe, and this gas is as needful for the germination of
seeds, and the action of roots and leaves, as it is for our maintenance of
life. It is hurtful for plants to be kept with too much water about their
roots, because this keeps out the air. This is the reason why house-plants
are injured if they are kept too wet.

A secondary office of root-hairs is to aid the roots of seedlings to enter
the ground, as we have before noticed.

The root-hairs are found only on the young parts of roots. As a root grows
older the root-hairs die, and it becomes of no further use for absorption.
But it is needed now for another purpose, as the support of the growing
plant. In trees, the old roots grow from year to year like stems, and
become large and strong. The extent of the roots corresponds in a general
way to that of the branches, and, as the absorbing parts are the young
rootlets, the rain that drops from the leafy roof falls just where it is
needed by the delicate fibrils in the earth below.[1]

[Footnote 1: Reader in Botany. VI. The Relative Positions of Leaves and
Rootlets.]

5. _Comparison of a Carrot, an Onion, and a Potato_.--It is a good
exercise for a class to take a potato, an onion, and a carrot or radish to
compare, writing out the result of their observations.

The carrot is a fleshy root, as we have already seen. The onion consists
of the fleshy bases of last year's leaves, sheathed by the dried remains
of the leaves of former years, from which all nourishment has been drawn.
The parallel veining of the leaves is distinctly marked. The stem is a
plate at the base, to which these fleshy scales are attached. In the
centre, or in the axils of the scales, the newly-forming bulbs can be
seen, in onions that are sprouting. If possible, compare other bulbs, as
those of Tulip, Hyacinth, or Snowdrop, and the bulb of a Crocus, in which
the fleshy part consists of the thickened base of the stem, and the leaves
are merely dry scales. This is called a _corm_.

The potato is a thickened stem. It shows itself to be a stem, because it
bears organs. The leaves are reduced to little scales (eyelids), in the